The invention relates to an electromagnetic wave detector and, more particularly, to a wave detector using semiconductors with quantum wells.
At present, several approaches may be used to detect an electromagnetic wave. These approaches include the use of:
1) Semiconductor materials with appropriate forbidden band widths; PA1 2) Doped semiconductor materials; PA1 3) Associated III-V semiconductor materials with multiple quantum well structures. PA1 B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker and R. J. Malik, "New 10 .mu.m infrared detector using intersubband absorption in resonant tunnelling GaAsAs superlattices", APL 50 (16), Apr. 20, 1987, p. 1092; PA1 K. K. Choi, B. F. Levine, C. G. Bethea, J. Walker and R. J. Malik, "Multiple quantum well 10 .mu.m GaAs/Al.sub.x Ga.sub.1-x As infrared detector with improved sensitivity", APL 50 (25), Jun. 20, 1987, p. 1814. PA1 B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker and R. J. Malik, "Quantum well avalanche multiplication initiated by 10 .mu.m intersubband absorption and photoexcited tunnelling", APL 51 (12), Sep. 21 1987, p. 934. PA1 B. F. Levine, C. G. Bethea, G. Hasnain, J. Walker, R. J. Malik, "GaAs/AlGaAs quantum-well long-wavelength infrared (LWIR) detector with a detectivity comparable to HgCdTe", Elec. Letters, Vol. 24, No. 12, Sep. 6, 1988, p. 747. PA1 B. F. Levine, C. G. Bethea, G. Hasnain, J. Walker and J. Malik, in "High Detectivity D*=1.0.times.10.sup.10 cm Hz/W GaAs/AlGaAs multiquantum well=8.3 .mu.m infrared detector", APL 53 (4), Jul. 25, 1988, p. 296. PA1 In the case of the configuration of FIG. 1a, the transitions occur between two levels located beneath the potential barrier separating the wells. Consequently, the electrons located on the two levels show a high degree of confinement in the width d.sub.1 of the wells, whence a high transition efficiency between the two levels. For, the absorption due to the electron transitions between e.sub.1 and e.sub.2 is one of the elements governing the response of the detector. This absorption is proportional to the interaction dipole .mu..sub.z between the two levels. .mu..sub.z is written: ##EQU1## where .PSI..sub.1 (z) and .PSI..sub.2 (z) are the real electron wave functions attached respectively to the levels e.sub.1 and e.sub.2. Now, for a given well thickness, .mu..sub.z will be all the higher as the extensions of the two wave functions are close to each other. The working of this type of detector takes advantage of the difference in mobility in the direction perpendicular to the plane of the layers constituting the wells, between the electrons located at the level e.sub.1 and those located at the level e.sub.2. For, in order to be extracted from the well, the electrons located at the level e.sub.2 have a potential barrier .PHI. to be crossed that is lower than the electrons located at the level e.sub.1 retained, under the effect of a field, in the well by a barrier .PHI.+h.upsilon.. Furthermore, the width of this barrier is smaller (1.sub.2) for the level e.sub.2 than for the level e.sub.1 (1.sub.1), and this is so because of the potential profile communicated to the structure by the electric field used for the collection of the photo-electrons. Nevertheless, this potential barrier that has to be crossed limits the performance characteristics of the detector, for it restricts the difference in mobility in the transfer perpendicular to the semiconducting layers, between the electrons located at the level e.sub.1 and those located at the level e.sub.2, the two levels being separated by the energy h.upsilon. given by the central wavelength of the response curve of the detector. PA1 If we now look at the structure of FIG. 1b, it is seen that the electrons excited by the electromagnetic field to be detected no longer have any barrier to cross in order to be extracted from the well: this favors their participation in the photocurrent. By contrast, the level to which the photoexcited electrons are taken from the level e.sub.1 is not bound, and the probability of transition is far smaller than in the case of FIG. 1a, taking into account the very low localization, in the zone of the well, of the electrons located at this free level. In other words, the wave function associated with this level is not confined to the zone of the well, and the interaction dipole .mu..sub.z is far smaller. PA1 a stack of layers of semiconducting material constituting a quantum well, the asymmetrical potential profile of which shows a stepped barrier, namely it has an intermediate barrier step, said stack possessing two energy levels, one of which is lower than the energy level of the intermediate barrier step while the other level is higher than the energy level of this intermediate barrier step; PA1 means to apply an electric field to the structure; PA1 means to detect an electric current that are connected to the terminals of the structure; PA1 a) a semiconducting structure possessing at least one stack of a first layer, second layer, third layer and fourth layer, the forbidden band widths of which make it possible to obtain the following profile of potential energy corresponding to the bottom of the conduction band for the electrons: PA1 the lowest energy for the second layer; PA1 intermediate energy for the third layer, this third layer constituting an internal barrier; PA1 higher values of energies for the first and fourth layers constituting the barriers of the wells; said structure being such that: PA1 the energy corresponding to a first permitted electron level is lower than the intermediate energy of the bottom of the conduction band of the material of the third layer; PA1 the energy corresponding to the second electron level is between the intermediate energy of the bottom of the conduction band of the material of the third layer and the potential energies of the bottom of the conduction band of the materials of the first and fourth layers; PA1 b) means to apply an electric field to the structure, oriented in the direction going from the third layer towards the second layer; PA1 c) means to detect an electric current that are connected to the terminals of the structure.
The first approach (1) consists in the use of semiconductor materials whose forbidden band width is smaller than the photon energy h.upsilon. of the wave to be detected, thus enabling the electrons to go from the valence band to the conduction band. These electrons are then collected by means of an external circuit, and are the source of a photocurrent that enables detection.
The second approach (2) relates to the use of semiconductor materials with a greater forbidden band width than the photon energy to be detected, which can be done by resorting to a doping of the materials used. This doping can be used to bring out an electron donor level corresponding to the impurities that are the source of the doping. From this energy level, which is closer to the conduction band than is the top of the valence band, it will be possible to produce electron transitions towards the conduction band so that the electrons, that have undergone these transitions under the effect of an electromagnetic field, are made free and detectable.
The third approach (3) is based on the occurrence of electron transitions between permitted energy levels (e.sub.1 and e.sub.2) within the conduction band of semiconducting quantum structures. FIG. 1a shows an example of this type of transition in a well exhibiting two permitted discrete energy levels for the electrons. Through the application of an electrical field to this type of configuration, it is the electrons located at the second quantum level that will tend to be extracted. Thus the collection, in the external electrical circuit, of these electrons coming from the second quantum level to which they have been taken by an illumination (h.upsilon.) enables the detection of this illumination. The invention relates to a detector corresponding to this third approach.
The type of detectors to which the invention can be applied is therefore based on the occurrence, under the effect of an illumination h.upsilon., of electron transitions within the conduction band of the semiconductor quantum wells. The principle of the working of these detectors is therefore that of using these transitions to place the electrons, initially located at the fundamental level of the well, at an energy level that enables them to leave the well easily under the effect of an applied electric field. Up till now, two configuration have been proposed to make this type of operation possible. A first configuration consists in the use of a quantum well with two permitted energy levels e.sub.1 and e.sub.2 for the electrons in the conduction band. As shown in FIG. 1, under the effect of an illumination represented by its photon energy h.upsilon., electron transitions may take place from the level e.sub.1 towards the level e.sub.2. The application of an electric field E to the structure then enables the electrons located at the level e.sub.2 to be made to cross the potential barrier .PHI. so that they are extracted from the well. The electrons then take part in a photocurrent that enables the detection of the illumination as is described in the following documents:
A second configuration is based on the use of a well having only one level, separated from the top of the barrier forming the well by an energy close to the photon energy h.upsilon. of the electromagnetic energy to be detected as shown in FIG. 1b and described in the following documents:
To obtain high absorption of the illumination to be detected, a large number of wells is used within the detectors based on this quantum principle, and this is done whatever the chosen configuration. Thus, the conduction band of these multiple quantum wells (MQWs) may be symbolized by FIGS. 2a and 2b corresponding respectively to the two types of transition that have just been referred to. This type of structure is based on the periodic stacking of layers of a first material M.sub.1 and a second material M.sub.2. The forbidden band widths of the two materials are different in order to obtain potential wells for the electrons in the conduction band. The contact layers located on each side of the periodic region designated by MQW may be constituted, for example, by layers of material M.sub.1 with high n type doping, referenced m.sub.1 /n+. To increase the number of possible transitions, an n type doping of the zone MQW is generally resorted to.
Among the characteristics exhibited by these configurations, we may note the following elements in referring again to FIGS. 1a and 1b:
Again, as a point of comparison between the two known configurations of FIGS. 1a and 1b, it may also be pointed out that, in the case of FIG. 1a, the distance 1.sub.2 depends directly on the field E applied to the structure. The more it is sought to diminish 1.sub.2, the more is it necessary to apply a strong field E and the more is 1.sub.1 diminished, the consequence of which is to increase the dark current of the structure. This problem is different in the case of FIG. 1b, where the weakest fields may be enough for the efficient collection of the photoexcited carriers but where, as we have seen, there arises the problem of the low probability of transition between the fundamental level of the well and the excited virtual level. To associate, on the one hand, a high response due to a high probability of transition between a fundamental level and an excited level and, secondly, a very low dark current, an approach using symmetrical wells with double stepped barrier, as shown in FIG. 3, has been proposed in the French patent application No. 89 08961. This approach is based on the differences between the extension of the wave function .PSI..sub.2 attached to the level e.sub.2 and that of the wave function .PSI..sub.1 attached to the level e.sub.1, to favor the the extraction from the well of the electrons located at the level 1 while, at the same time, preserving a high degree of confinement for the electrons located at the level 1. However, it is possible to even further promote this behavior by using asymmetrical wells as described here below.
To resolve the problem raised, namely to obtain a high response and a weak dark current on one and the same detector based on the occurrence of intraband transitions within the conduction band of multiple quantum well structures, the barrier to be crossed has to be very different for the electrons located on the level excited and for those located on the fundamental level.